The present invention relates to a compressor unit for the production of compressed air, of the type in which a rotary compressor draws air from an intake opening and directs it to a pressure tank through a minimum-pressure valve.
Known units of this type are presently made by a method now consolidated by the vast experience of a multitude of manufacturers all over the world. Although there are obvious differences in individual details, all the compressor units reflect a common design which will now be explained in general terms.
In known units the compressor is of the rotary volumetric type, typically with radial vanes, driven by an electric motor; these compressors draw in air from the atmosphere through an intake opening and direct it to a pressure tank through a minimum-pressure valve which opens only when a predetermined minimum pressure has been reached within the compressor unit.
It is well known that particularly accurate lubrication is of vital importance to the operation of vane compressors, not only for the bearings of the rotor but also within the casing to limit friction between the vanes and the casing itself, in order simultaneously to ensure sealing and to facilitate heat exchange which reduces the temperature and enables the efficiency of the compressor to be improved.
For this lubrication, oil supplied by the pressure of the air itself downstream of the compressor is used. To this end, a container which is partially filled with oil is provided between the compressor and the minimum-pressure valve; the pressurised air acts on the free surface of the oil so that the oil is also at the pressure of the air. The pressurised oil is then supplied to the compressor through suitable ducts and reaches the rotor bearings and the interior of the casing to ensure the necessary lubrication.
The operation of such a compressor unit under running conditions should be completely satisfactory but it is well known that other measures are necessary for starting to be possible without risk.
In fact, if the rotor were to stop suddenly and cut off the flow of air, the pressure in the container would cause oil to flow into the casing, filling the compartments between the vanes; on subsequent starting this oil, which is practically incompressible, could easily cause damage or breakage of the compressor.
In order to avoid this, the pressure in the container is reduced before the compressor is stopped. For this purpose, the container is provided with a discharge valve which is norma11y kept closed by a spring and the opening of which is controlled pneumatically by a pilot circuit.
In practice, the stoppage of the motor-compressor unit includes a first emptying stage and a second stage in which the rotary compressor is stopped.
In the emptying stage, the pilot circuit is connected to the pressure tank by a solenoid valve and causes:
1. the closure of the minimum-pressure valve by suitable piloting, PA0 2. the opening of the discharge valve against the action of the spring, PA0 3. the closing of the intake opening by means of a slide.
The pressure in the container is then lowered to atmospheric pressure and while the rotor continues to rotate creates a low pressure between it and the closed intake opening; this low pressure is used to draw the oil from the lubricating ducts of the rotor and the casing by means of a suitable transfer port in the slide which opens with the closure of the intake opening.
In this way, all the oil ducts close to the compressor are emptied and after a suitable period of time this is stopped. This stoppage is effected by a delay circuit which is activated together with the solenoid valve. When the rotor has stopped, there is no longer any pressurised oil which could fill the compartments between the vanes.
The subsequent starting can therefore take place without risk, the pressure in the pilot circuit being relieved and the rotor being started. The pressure of the air and oil in the unit increases progressively and running conditions are reached.
In addition to what has been explained, in order to allow the emptying cycle to be carried out even when the compressor unit is not connected to a pressure tank (for example during testing), the known units normally also include a so-called selector valve whereby the pilot circuit may be put into communication with the interior of the pressure container instead of with the tank. This valve is generally of an automatic or self-piloting type in accordance with a technique well known in the art.
As confirmed by its almost universal use, the design described is judged satisfactory and, as seen, effectively solves the normal problems of stoppage and starting of the machine.
In reality, these problems are solved only in the case of a desired stoppage, however, not in the case of an accidental stoppage due, for example, to unexpected power cuts. Even in this situation emptying can occur as long as the solenoid valve has a biassing spring which opens it in the absence of electrical power. In all cases, however, the stoppage of the rotor is almost simultaneous and the effectiveness of the emptying is much reduced. In particular, the pressure within the container does not reach zero before the rotor stops, which thus causes both a leakage of oil between the vanes and a sudden counter-rotation of the rotor with the consequent expulsion of a spray of an oil-air mixture through the intake opening before it has been closed completely by the slide. This phenomenon is obviously very troublesome since it fouls any filter on the intake and, in any case, the risk of damage to the vanes on subsequent starting remains.